It is well known that solar cells or photovoltaic cells can be used to convert solar energy into electric current. Typical photovoltaic cells include a substrate for mounting the cell and two ohmic contacts or conductors for passing current to an external electrical circuit. The cell also includes two or three semiconductor layers in series. The two layer type of semiconductor cell consists of an n-type layer and a p-type layer, and the three layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers applies a net voltage across two electrode layers in the photovoltaic cell, establishing a current of electricity.
The semiconductor layers may be formed from single crystalline materials, amorphous materials, or polycrystalline materials. Single crystalline layers are often made with a molecular beam epitaxy (MBE) process (or other vapor deposition process), but the largest area of a substrate that can be practically covered using such processes is on the order of several square centimeters because it is limited by the size tri of single crystal substrates, which is an impractical size when considering the surface area required for economically practical solar cells. Therefore, although single crystal photovoltaic materials can be used to generate conversion efficiencies over 20 percent, they have significant drawbacks because of their high manufactured cost. Accordingly, where the solar cell must compete with conventional electricity generation by nuclear or fossil fuel, polycrystalline materials are viewed as the material of choice for the production of semiconductors and solar cells using such semiconductors. Typically, the polycrystalline material of choice for a semiconductor in a photovoltaic cell is a group II-group VI compound, such as cadmium telluride. Cadmium telluride is preferred for thin film photovoltaic applications because of its direct band gap and its ability to be doped n-type and p-type, which permits formation of a variety of junction structures. It is known that an RF sputtering technique can be used to deposit thin films of cadmium telluride onto substrates for use in photovoltaic cells, as disclosed in U.S. Pat. No. 5,393,675 to Compaan. The RF sputtering technique can also be used for depositing other thin group II-group VI semiconductor films such as cadmium sulfide and zinc telluride for use in a photovoltaic cell. RF sputtering involves positioning a substrate in a pressure chamber and operating a magnetron sputtering gun. The gun includes a target (the cathode) of pressed and sintered cadmium sulfide or cadmium telluride typically prepared from powder. The substrate is positioned behind the target and is coated as the target is bombarded. The process takes place typically in an inert atmosphere of argon gas.
In most photovoltaic cells it is necessary to dope one or more semiconductor layers to be highly conductive to achieve easy flow of electrons and holes into the respective contact electrodes. Particularly for cadmium telluride and zinc telluride and related semiconductors, copper is often used for this dopant. While the doping with copper is successful in obtaining the desired conductivity, the use of copper has its limitations. It has been found that over time the copper diffuses into other semiconductor layers of the photovoltaic cell, thereby causing a loss in efficiency. When copper is used to dope a zinc telluride contacting layer the copper tends to move into the cadmium telluride layer and even penetrate into the cadmium sulfide/cadmium telluride junction where it degrades the photovoltaic activity. Further, when zinc telluride and other semiconductors are heavily doped with copper, the semiconductor layer begins to lose its transparency to radiation transmission.
There remains a continuing need for a more efficient and less expensive photovoltaic cell. It would be advantageous if there could be developed a contacting layer material suitable for solar cells, where the material minimizes or eliminates the problem of diffusion of copper into other layers of the cell. Further, it would be helpful if such a contacting layer could be transparent to solar energy in wavelengths that are not absorbed by the upper semiconductor layers such as cadmium sulfide and cadmium telluride so that capture of additional energy could be made in a second solar cell underneath the top solar cell.